Computer can't POST, turn off - flashing power, keyboard lights

Problem description:
I built my computer in January 2008. When it is turned on, there is a blue light on the front panel next to the power button that burns steadily (Case is from Cooler Master).

Most nights I turn my computer off when I go to bed. Sometimes though I don't and leave it running all night. The longest I've ever had it on non-stop is probably about 72 hours. After about 30 minutes my monitor goes on standby and switches off. It wakes up and returns to desktop when I press a button or move the mouse.

Last night I kept it on. It had been off the previous night so it was probably now running for about 20 hours. When I returned to it this morning I moved the mouse and pressed some buttons and nothing happened.

I tried turning the monitor on and off but it remained on standby.
So I decided to restart the computer. I pressed the restart button on the front panel but nothing happened. I held down the main power button for 20 seconds and nothing happened. At this point I noticed that the blue light was flashing instead of steadily burning.
The only way I could turn it off was by turning off the psu power button at the back.

I can not get the POST screen to appear. In summary, I can not get my computer to boot up when I turn it on. I can only turn it off by disconnecting the power.

When I press the power button, the blue light flashes; and the numlock, capslock and third light on my keyboard flash as well. The monitor does not turn on and there are no beeps emitted by the machine. The DVD tray does not respond to the eject button. The only thing that occurs are that the psu fan, graphics card fan, processor fan and case fan all engage but the volume of the noise they make pulsates in rhythm to the flashing lights. The fan for the graphics card takes a while to get going, stopping a couple of times at first.

Additionally a green LED on the bottom right hand of the mobo labled SB PWR lights up whenever the psu power switch is turned on.

Attempted fixes:

1) Cleaned dust out of the internal components and double checked all connectors are secure
2) Removed the power cord and pressed the power button for 30 seconds
3) Exchanged the external power cord, and internal power cord for the graphics card with spare ones.
4) Unplugged all hardware and disconnected all components from the mobo apart from graphics card, psu and monitor.
5) Removed RAM sticks, cleaned slots and swapped them around; tried only using 1 RAM stick.
6) Removed the mobo from from the case and repeated 3)
7) Removied the CMOS battery from the board and used the reset jumpers to clear the BIOS settings

All to no avail. Exact same problem persists.

Recent changes:
The only change I made the day before the computer stopped functioning was that I accessed msconfig and stopped it from disabling any startup processes by selecting Enable All. I restarted the computer. It worked fine.

Operating system: Windows XP Home. 32-bit.

System specs:
Processor: Intel Core 2 Duo E6320
Motherboard: Asus P5B
Graphics card: Nvidia XFX GeForce 8800 GTS
Memory: 2x elixir 1GB
Power supply: Cooler Master Real Power M620. Total output: 620W
Hard drive: Seagate Barracuda 500GB

Googling the problem has led me to suspect that my psu might be broken somehow but otherwise I have no clue how to test it.
Could anyone give me some advice for what to do next?
1 answer Last reply
More about computer post turn flashing power keyboard lights
  1. Work through our standard checklist and troubleshooting thread:
    I mean work through, not just read over it.

    Breadboard - that will eliminate any kind of case problem you might have.

    Breadboard with just motherboard, CPU & HSF, case speaker, and PSU. You do have a case speaker installed, right? If not, you really, really need one. If your case or motherboard didn't come with a system speaker, you can buy one here:

    You can turn on the PC by momentarily shorting the two pins that the case power switch goes to.

    You should hear a series of long, single beeps indicating memory problems.
    Silence indicates a problem with (in most likely order) the PSU, motherboard, or
    CPU. Remember, at this time, you do not have a graphics card installed so the load on your PSU will be reduced.

    If no beeps:
    At this point, you can sort of check the PSU. Try to borrow a known good PSU. If you cannot do that, use a DMM to measure the voltages. Measure between the colored wires and either chassis ground or the black wires. Yellow wires should be 12 volts. Red wires: +5 volts, orange wires: +3.3 volts, blue wire : -12 volts, violet wire: 5 volts always on. Tolerances are +/- 5% except for the -12 volts which is +/- 10%.

    The gray wire is really important. It should go from 0 to +5 volts when you turn the PSU on with the case switch. CPU needs this signal to boot.

    You can turn on the PSU by completely disconnecting the PSU and using a paperclip or jumper wire to short the green wire to one of the neighboring black wires.

    This checks the PSU under no load conditions, so it is not completely reliable. But if
    it can not pass this, it is dead. Then repeat the checks with the PSU plugged into the computer to put a load on the PSU.

    If the system beeps:
    If it looks like the PSU is good, install a memory stick. Boot. Beep pattern should
    change to one long and several short beeps indicating a missing graphics card. Silence or long single beeps indicate a problem with the memory.

    Insert the video card and connect any necessary PCIe power connectors. Boot. At this point, the system should POST successfully (a single short beep). Notice that you do not need keyboard, mouse, monitor, or drives to successfully POST.

    Now start connecting the rest of the devices starting with the monitor, then keyboard and mouse, then the rest of the devices, testing after each step.
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